1. Field of the Invention
This invention relates to a low melting glass having a low refractive index and an absorption in infrared regions for glass laser coating.
2. Description of the Prior Art
Recently, it was reported that the oscillation characteristics of a laser glass disc could be improved by coating the outer side of the laser glass disc with a thin layer of glass, thereby reducing the light scattered by the inside wall of the disc, and absorbing the light by the thin layer of glass.
Sealing glasses used for this purpose must have the following characteristics. That is, the sealing glass must have an expansion coefficient, .alpha., equivalent to that required in a conventional sealing glass and the glass must be capable of sealing at a temperature lower than the transition temperature, Tg, of the laser glass in order to not change the characteristics of the laser glass at sealing. Furthermore, a sealing glass must absorb light having a definite wave length (e.g., 1.06 microns in the case of a glass laser where the glass contains Nd) and further must sufficiently meet refractive index requirements. For example, in order to prevent reflection at the inside wall of a Nd laser glass disc, the sealing glass used must, as a matter of course, have absorbtion at 1.06 microns and further for effectively preventing reflection, it is believed to be better that the refractive index of the sealing glass be higher than the refractive index (1.55-1.60) of a conventional silicate laser glass and also the difference be as low as possible (a difference of less than 10%). Moreover, as the size of a laser glass disc increases, the amount of light passing through the glass increases, and hence the difference between the refractive index of the laser glass and that of the low melting glass must be small (a difference of less than 5%). Still further, since a laser glass having a lower refractive index (1.54-1.53) is required to reduce the self-focusing effect of the laser glass, the low melting glass used as the sealing glass must also have a lower refractive index.
Therefore, in order that the characteristics of a laser glass at sealing not be changed, the sealing glass used must, as a matter of course, have a sealing temperature lower than the transition temperature, Tg, of the laser glass.
Ordinary low melting glasses used conventionally for this purpose are substantially B.sub.2 O.sub.3 --Tl.sub.2 O--ZnO glasses and B.sub.2 O.sub.3 --PbO--ZnO glasses and if these glasses are modified to have a softening point (Sp) of about 400.degree. to 500.degree. C and an expansion coefficient of 100 .+-. 20 .times. 10.sup.-7 /.degree. C, it is assumed that the refractive index (nd) of these glasses will be higher than 1.8 to 1.9, which is a drawback of these conventional low melting glasses.